In prior art semiconductor lasers, as typically disclosed in U.S. Pat. No. 5,172,384, entitled "Low Threshold Current Laser", issued Dec. 15, 1992, it is common to form an active region with an indium gallium arsenide quantum well, having guiding layers and cladding layers of aluminum gallium arsenide on both sides thereof. Further, it is typical to form Bragg reflectors, or mirror stacks, on each side of the active region, which mirror stacks generally include aluminum gallium arsenide. The epitaxial configuration of VCSELs is well defined, however, various processing schemes can be utilized.
The most commonly used structure involves proton implantation in order to separate individual devices on a substrate. The advantage of this structure is its planar character; it suffers, however from instability of optical lateral mode and relatively high threshold currents due to the large current spreading.
In instances where the semiconductor laser is to be a ridge waveguide, or patterned mirror, laser, the upper mirror stack is etched to the correct depth by knowing the exact rate of etch and timing the etch process. This prior art method of etching the upper mirror stack is very difficult and inaccurate. Thus, the prior art patterned mirror VCSELs have relatively low threshold currents but lack the needed reliability.
In some instances, such as in a copending application entitled "VCSEL With Vertical Offset Operating Region Providing a Lateral Waveguide and Current Limiting and Method of Fabricating", U.S. Pat. No. 5,351,257, filed Mar. 8, 1993, including an etch-stop layer in the upper mirror stack has been proposed to automatically stop the etch at a desired depth.
In a U.S. Pat. No. 5,293,392, entitled "Top Emitting VCSEL with Etch Stop Layer", issued Mar. 8, 1994 an etch stop layer is grown in the second, or upper, mirror stack and utilized to automatically stop the etch at a desired level. While the etch stop method utilizes the excellent control achieved in MOVPE growth to define the patterned mirror height, the growth of a layer of dissimilar material (the etch stop layer) is required which adds some complication to the fabrication process.
To provide such an etch-stop layer, a layer from a different material system must be epitaxially grown between layers of the normal or desired material system. Thus, epitaxial layers of the normal or desired material system are grown to a desired height. The epitaxial growth is then stopped and restarred with the different material system until the etch-stop layer of desired thickness is grown. The original epitaxial growth is then continued until the device is completed. Generally, this is difficult and requires substantial effort and time to complete.
In a copending application entitled "VCSEL With Al-Free Cavity Region", filed of even date herewith, An aluminum-free active region is utilized as the etch stop. However, in some instances it is desirable to terminate the etching of the upper mirror stack before the active region is reached.
Accordingly, it is desirable to provide a method of easily and accurately etching the upper mirror stack of a VCSEL.
It is a purpose of the present invention to provide a method of easily and accurately fabricating patterned mirror VCSELs.
It is a further purpose of the present invention to provide a method of fabricating VCSELs which is relatively easy to perform and which accurately stops the etching process before damaging the active region.
It is a further purpose of the present invention to provide a method of fabricating VCSELs which results in VCSELs that are highly reliable, relative to prior art VCSELs.
It is a further purpose of the present invention to provide VCSELs which are highly reliable, relative to prior art VCSELs.